Television keystone balance control circuit



Dec. 1, 1953 H. R. SMITH 2,561,443

TELEVISION KEYSTONE BALANCE CONTROL CIRCUIT Filed D60. 30', 1949 bOK/ZO/VTAL SIG/VAL [2 SI sou/(=65 {6 VEPT/CAL SIG/VAL SOURCE Fig. 20 Fig. 2b

Fig. 3 a Fig. 3b

Fig. 4 b F I9. 4 a INVENTOR.

I HARRY R. SMITH Patented Dec. 1, 1953 TELEVISION KEYSTONE BALANCE CONTROL CIRCUIT Harry R. Smith, Glen Ridge, N. J., assignor to Allen B. Du Mont Laboratories, Inc., Passaic, N. J a corporation of Delaware Application December 30, 1949, Serial N 0. 136,092

3 Claims.

This invention relates to an electrical circuit for producing a predetermined pattern on the screen of a cathode ray tube. It relates particularly to circuits for distorting the raster on cathode ray tubes having off center electron guns such as iconoscope tubes, certain projection tubes, and tubes of the type disclosed in U. S. Patent 2,480,848.

Throughout this specification the term iconoscope type cathode ray tube will be used to denote any cathode ray tube having an off center electron gun.

The keystone pattern of iconoscope type tubes is familiar to those skilled in the art. To cause the pattern to assume a normal rectangular shape it is necessary to introduce correcting voltages into the deflection amplifiers. The correcting voltage for the horizontal amplifier may be a sawtooth voltage at the vertical frequency which is used to modulate the horizontal defleeting voltage before it passes through the horizontal output transformer. It is necessary for the horizontal output transformer to pass the comparatively low vertical frequency without distortion as well as harmonics of the horizontal frequency, up to about the tenth, in order that such a system may operate properly.

This invention consists in adding a controlled current, varying at the vertical frequency, to the horizontal deflecting current flowing through the horizontal deflecting coils.

An object of this invention is to provide an improving mixing circuit.

Another object is to provide an improved keystone correction circuit for use with cathode ray tubes having on" center electron guns.

Another-object is to correct for deficiencies in the deflection yoke of magnetically deflected cathode ray tubes.

Still another object is to compensate for the limited frequency response of transformers.

Further objects will be apparent to those skilled in the art after studying the following specification and drawings in which:

Figure 1, partly in schematic and partly in block diagram form, shows one embodiment of the invention;

Figure 2a shows a horizontal sawtooth current wave modulated unequally by a vertical sawtooth wave;

Figure 2b shows the pattern on the face of an iconoscope type cathode ray tube when a deflection current such as that shown in Figure 2a is used;

Figure 3a shows the desired deflection current for an iconoscope type cathode ray tube;

Figure 3b shows the normal rectangular pattern produced on the screen of an iconoscope type cathode ray tube by a deflection current of the type shown in Figure 3a;

Figure 4a shows a horizontal sawtooth current wave with a vertical sawtooth wave added; and

Figure 4b shows the pattern produced on the face of a standard cathode ray tube when standard deflection currents are applied to a yoke in which the magnetic fields are not at right angles to each other.

In Figure l a horizontal transformer II is shown having its primary I2 connected to a source is of horizontal voltage, and its secondary It connected to one end of a horizontal deflection coil IS. The other end of the secondary It is connected to a tap I! on the keystone balance control I3. The contact arm IQ of this potentiometer is connected to the contact arm 2I of the horizontal positioning control 22. A tap 23 on this control 22 is connected to the other end of the horizontal deflection coil It. The control 22 is connected across a source 24 of horizontal positioning current which is shown here as a battery.

The keystone balance control I 8 is connected across the secondary 26 of a transformer 21, which maybe an audio transformer having its primary 28 connected to a vertical deflection sawtooth signalgenerator 29. A capacitor 3| is connected from the center tap I I of the keystone control I8 to the center tap 23 of the horizontal positioning control 22 to provide a low impedance path between these points for the high frequencies in the horizontal deflection wave.

Under normal operating conditions of the circuit in Figure '1 the horizontal voltage source I3 presents a deflection voltage of the type indicated by reference character 32, to the primary winding I2 of the output transformer I I. This voltage wave 32 is preferably an equally modulated horizontal deflecting wave, so that in passing through the transformer II it will not be distorted and the resulting current wave 33, as shown in Figure 2a, flowing through the secondary It of the transformer II will also be symmetrical. This wave, flowing through the horizontal deflection coils I6, would normally be expected to produce a pattern such as is shown in Figure 2?) on the face of the iconoscope type tube. However, non-symmetrical yoke or tube construction frequently results in a raster such as is shown in Figure 3b.

The vertical signal source 29 causes a substantially sawtooth current wave 34 to flow in a secondary 26 of a transformer 21. If the contact 19 on the potentiometer I8 is rotated so as not to be electrically contiguous with the tap I1, a portion of the current 34 will be diverted to flow through the secondary l4 and the coils IS. The amplitude and direction of flow of the diverted portion of the current 34 are determined by the direction and amount of rotation of the contact l9 from the zero point; i. e., the point at which it makes direct electrical contact with the tap H. The addition of the current 34 to the equally modulated current wave 3.3 shown in Figure 2a results in the non-symmetrical current wave I33 shown in Figure 3a. If the proper amplitude and. polarity of the current 34 is added to the cur:- rent 33, the resultant current 33 will counter:- act the non-symmetrical yoke or tube construc-' tion and produce the desired, rectangular raster as shown in Figure 2b.

Similarly the amount and direction of the D. C- p itiohing current f wi through th co s 16 due to the direct current flowing through the potentiometer 22 are controlled by the rotation of the contact 2| with respect to the tap 23.

The highfrequencies ofth'e horizontal deflecting current are bypassed by means of the condenser ,31 and do not flow' through the potentiometers 22 and I8.

Although the potentiometer I8 is shown with a tap ll intermediate the ends thereof, this is a matter of convenience only since, if the desired polarity of the additional current through the coils I6 derived from the current 34 is known, it would be possible to connect one end of the sec.- ondary M to one end of the potentiometer I8 instead of to the tap ll. In such case, only the amplitude of the additional current through the coils [6 could be controlled by the rotation of the contact IS). The polarity would always remain the same. 'Ihe positioning circuit comprising the potentiometer 2g and the voltage source 24 has been inserted for purposes of illusn h known cir ui elem nt and ar no necessary to the operation of this invention. For instance, if no positioning were necessary, the co a 9 ld be c nnected i ect y t t junction of the condenser 3! and the coils H5.

The raster in Figure 4b is the result of having the cal deflec ion soi s rot t kwise ias viewed m h fa e o a standar cath d ray tube) w h e e t t the hori ontal deflection i s Th ci cu t of F gure m y be used to overcome is t pe of dis ort on iust as it was u t n era t t key tone distortion. :In s case e ol a e wave 32 supplied by the zo l si na sou e 31s not modulated and the amplitude of each of the horizontal voltage 4 oscillations is the same. The current wave-form through the coils I6 is shown in Figure 4a with the vertical saw component added.

It will be apparent that modifications of the invention may be made, the scope thereof being definedby thefollowing claims.

What is claimed is:

1. In a television deflection circuit for a cathode ray tube having an off center electron gun, a source of horizontal deflecting voltage, a horizontal output transformer havin a primary and "a secondary winding, said primary being connected to said source of voltage, a horizontal deflectioncoil, one end of said deflection coil being connected to a first tap on said secondary winding, a source of vertical deflection voltage, a vertical output transformer having a primary and a ,s ecgndary winding, said primary of said vertical transformer being connected to said source of ver ical de l ction v l a an i pedance co nected n ser es with sa d s da y wind n o said vertical output transformer, a second tap on said se o r of said hor n al output transformer being connected to a point on said impedance, the other end of said deflection coil being connected to another point .on said impedance.

2. The circuit of claim 1 in which one of said points on said impedance is variable with respect to the other oisoid points.

' 3. In a television deflection circuit, a plurality of sources of deflection voltages, a first transfprmer having a primary and a secondary winding, said primary windingbeing connected to a first said source of deflection voltage, a deflection coil, one end of said coil being connected to a first ta on a d s cond ry n i a s ond transformer having a primary and a secondary Winding, said primary of said second transformer being connected to a secon Said source of deflection voltage, an impedance connected in series with said secondary winding of said second transformer, a second tap on said secondary of said first transformer connected to a point on said impedance, the other ,end of said deflection coil being connected to another point on said is es-dens sunfi- References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,485,727 Sindeband Mar. 4,1924 2,477,557 Torsch July 26, 1949 2,505,011 Smyth Apr. 25, 1950 

